Sharp edge orifice dashpot timer

ABSTRACT

The dashpot timer of this invention includes a substantially cylindrical glass tube in which travels a piston having a diameter slightly less than that of the interior of the tube. A gas defines the medium in the cylinder in which the piston containing a sharp edge orifice is adapted to travel. The flow ingenerated in the cylinder is predominately an inertial flow through the sharp edge orifice with the shear flow and pressure flow between the piston and cylinder being relatively insignificant. The device of the present invention operates in the inertial flow region whereby the viscosity effects are relatively small compared with the inertial effects of the fluid.

United States Patent 1 Breed 1*Dec. 11, 1973 SHARP EDGE ORIFICE DASIIPOTTIMER [76] Inventor: David S. Breed, Box 270, RD. 2

Hillcrest Rd., Boonton Twp., NJ. 07005 Notice: The portion of the termof this patent subsequent to Feb. 16, 1988, has been disclaimed.

[22] Filed: Feb. 16, 1971 [21] Appl. No.: 115,490

[52] 11.8. CI. 188/317, 58/144 [51] Int. Cl. F16g 9/02 [58] Field ofSearch 188/316, 317; 58/144; 16/66 [56] References Cited UNITED STATESPATENTS 2,714,927 8/1955 Stern et al. 58/144 3,553,959 1/1971 Young etal. 58/144 2,054,466 9/1936 Peo 188/317 X 3,043,404 7/1962 Peras v188/317 X 3,171,245 3/1965 Breed 58/144 3,418,880 12/1968 Herlach188/317 X FOREIGN PATENTS OR APPLICATIONS 309,066 9/1929 Great Britain188/317 Primary Examiner-George E. A. Halvosa Attorney-Kane, Dalsimer,Kane, Sullivan & Kurucz [57] ABSTRACT v the inertial flow region wherebythe viscosity effects are relatively small compared with the inertialeffects of the fluid.

9 Claims, 4 Drawing Figures SHEEI 10F 2 PMENIED um 1 1 new\\\\\\&A\\\\\\\L FIG.

FIG?) ATTORNEY PATENIEDBECH ms 3.777.861 SHEET 20? 2 FIG. 4-

INVENTOR DAULo '5. BREED KANEDAL$\MER,KANE|$4JLLNAN Kuamcz ATTORNEY 1SHARP EDGE ORIFICE DASIIPOT TIMER The subject invention relates to a gassharp edge orifice dashpot which utilizes the clearance between anaxisymmetirc piston and an interior cylindrical wall as the seal betweenthe piston moving relatively to the cylinder and containing a sharp edgeorifice through which the gas is metered.

Annular orifice dashpots utilizing air as the metering fluid are knownin the art and are described in U.S. Pat. No. 3,171,245. Such airdashpots are finding wide acceptance for certain time delays havingcertain functions of the applied force or where the available force iswithin certain narrow limits. Applications where the applied force issufficiently small usually being less than a few ounces and requiringcomplete temperature insensitivity or where it is desired to have thetime delay vary with the inverse square root of the applied force, orboth, air timers heretofore unavailable and unrealized are necessary.For such cases the gas sharp edge orifice dashpot timer of thisinvention has been found to be eminently satisfactory and acceptable andcapable of yielding time delays ranging from a few milliseconds toseveral minutes and more.

An understanding of the particular nature of the flow past a piston asit descends along the wall of a cylinder and through a sharp edgeorifice in the piston is important in determining the predictability andaccuracy of the rate of descent. The type of gas flow that the pres entinvention utilizes is generally termed inertial flow through the orificeand creeping, shear or pressure flow between the piston and cylinderwhich involves very slow gas motion between the piston and cylinder,that is, motion at very low Reynolds numbers and in which viscous forcesare predominant over inertial forces whereas the opposite is the casefor the flow through the sharp edge orifice in the piston where the gasvelocity is large yielding large Reynolds numbers and thus the inertialforces predominate over the viscous forces. The size of the orifice andpiston-cylinder clearance is such that the dominant portion of the flowoccurs through the sharp edge orifice with the small clearance betweenthe piston and cylinder thus acting as a seal.

It is, therefore, an object of the present invention to provide a gassharp edge orifice dashpot which eliminates the disadvantages,limitations and drawbacks of the prior art devices and which isexceptionally reliable, susceptible to long life and relativelyinexpensive to manufacture.

The gas dashpot of the subject invention has been successfully utilizedas a bomb retardation sensor system. In this application, the dashpotpiston is acted upon by the deceleration force due to the parachuteretarding the bomb and the piston mass. Since this force varies with thesquare of the bomb velocity and since the flow through the sharp edgeorifice varies as the square root of the applied force, the flow variesas the first power of the bomb velocity. This gives an approximateconstant safe separation distance between the bomb and the plane whenthe bomb becomes armed, for the case where the bomb is released from alevel flight. Although several approaches may be taken to Other objectsand advantages will become apparent from the following detaileddescription which is to be taken in conjunction with the accompanyingdrawings illustrating an exemplary preferred embodiment of the inventionand in which:

FIG. 1 is a diagrammatic perspective view of a dashpot timerincorporating the teachings of this invention utilizing a pistoncontaining the sharp edge orifice the initial and terminal position ofwhich for the prescribed time delay being shown in dotted lines;

FIG. 2 is an enlarged fragmentary longitudinal sectional view of thisdashpot showing the internally disposed piston traveling through theselected gas to an applied force;

FIG. 3 is an enlarged fragmentary sectional view of the piston showingthe sharp edge orifice in detail; and,

FIG. 4 is a longitudinal sectional view illustrating the dashpot timerapplied to a bomb retardation sensor.

Referring now to FIGS. 1 to 3, a dashpot timer of this invention willinclude an outer cylinder 2 having a contained gas 4 therein throughwhich a piston is adapted to travel under an applied force F. The pistonalso defines with the inner surface of the cylinders relatively smallannular clearance through which a small portion of the gas passes. Themajor portion of the fluid passes through the sharp edge orifice 8.

Referring now to FIG. 4, a bomb retardation sensor is shown generally at10. In order to seal the gas 12 in the cylinder 14, plates 16 and 18 areintegrally formed with or securely cemented to cylinder 14.

Upon release of the bomb and deployment of its drag chute, theretardation subjects the piston 20 to a deceleration causing it to begintraveling against a spring 22 at a rate determined by the flow of thegas 1 through the sharp edge orifice 23. When the piston reaches end 24it bridges contacts 26 and remains in this position due to end 18 whichis magnetic. Prior to bomb release the piston 20 was held against end 16by bias spring 22 where contacts 32 were bridged. Thus, the distancewhich the piston 20 has traveled along its axis provides a convenientmeasure of elapsed time and varies as the square root of the appliedforce. Through the use of additional circuitry the piston can beutilized to trigger various devices such as an arming mechanism 40 of abomb.

The motion of the piston in the cylinder depends upon the equation ofinertial fluid fiow through a sharp edge orifice providing the flow inthe clearance between the piston and cylinder can be neglected. Thisrelationship is:

where:

Q volume flow rate in./sec.

K experimental constant (usually about 0.7)

AP pressure drop across the orifice p density of gas A area of theorifice This equation holds for large Reynolds numbers where the viscouseffects can be neglected.

The'time delay for a given piston travel can be found from therelationships solving or if the force is independent of time where: r

L length of piston travel t= time delay f applied force A area of pistoncross section Since the viscosity does not appear in this equation, thetime delay will be independent of the viscosity insofar as theassumptions made above hold. That is, the Reynolds number must be muchlarger than 1, and the fluid flow between the piston and cylinder mustbe negligible. For a particular design air has functioned well from 65to 160 F with a force of 0.047 lbs. on the piston.

For most devices contemplated, the radius would be less than aboutone-half inch. The pressure could vary from about 0.1 psi to severalhundred psi, however, for most applications the pressure drop across theorifice would probably not exceed 20 psi. The clearance would probablynever exceed 0.001

For most applications, however, no temperature compensation would benecessary.

The fact that the time delay is proportional to the square root of theapplied force makes this timer ideal for application for constantdistance arming for rockets. For this case only one end of the cylinderwould be sealed.

The cylinder 14 is preferably made from glass or other ceramic material,since precision glass tubes are readily available. However, othermaterials could also be utilized. Similarly, the end plates 16 and 18could be made from glass, ceramic or metal such as aluminum. The platesmay be integrally formed with the cylinder or may be bonded to the endsof the cylinder.

In summary, the present invention accomplishes and contributes thefollowing advantages to the dashpot timer are:

1. Time delay as function of the square root of the applied force: Forall other fluid dashpots, the time delay will follow some other functionof applied force. To achieve constant safe separation distance armingfor retarded bombs, the time delay must vary as the inverse square rootof the applied force. This is accomplished through the inertial flowequation for a sharp edge orifice.

2. Temperature compensation: The viscosity of gases change by a factorof 1.5 over the temperature range 65 to +1 60 F. Even over a verylimited temperature range, the viscosity changes several percentConsequently, if accuracy is to be achieved even over limitedtemperature ranges the Reynolds number for the flow through the sharpedge orifice must be large compared to l.

3. Piston rates: For most of the devices of the present invention, therate of travel of the piston will be on the order of 0.005 to 50 inchesper second. The most important range effectively satisfied by thisinvention is the 0.1 to 25 inches per sec. area.

4. Sealing: The second major reason why gas dashpots have failed inmilitary applications has been the inability to seal the gas when a rodor other member must emerge from the timer. With the use of anelectrical output there is no sealing problem and a constant density ofthe gas can be maintained.

5. Military applications: Based on present knowledge, no gas inertialflow gas dashpot has been successfully applied to a military fuze in thepast. Other applications and other than one-time use dashpot timerapplications may be found in the above identified patent.

6. Piston cylinder seal: Based on present knowledge, no inertial flowgas dashpots have been made wherein the clearance between the piston andcylinder is the seal. Where orificedype dashpots have been utilized inthe past, in a piston-cylinder configuration, 0" rings or similar typeseals have been used or viscous flow has been used.

Although a preferred embodiment of this invention has been described andillustrated herein, it should be understood that this invention is in nosense limited thereby but its scope is to be determined by that of theappended claims.

I claim:

1. A gas, sharp edge orifice dashpot timer comprising:

a cylinder having a substantially cylindrical interior wall;

a piston disposed in said cylinder and having an outer diameter slightlyless that the diameter of said interior wall and containing a sharp edgeorifice whereby the major portion of dashpot gas flow occurs through thesharp edge orifice;

a gas in the cylinder through which the piston is adapted to move withthe relative flow of the gas being inertial flow through the orifice andflow in lubrication region between the piston and cylinder, whichinvolves Reynolds numbers much larger than 1 in the orifice so that theviscous effects can be neglected for flow through the orifice and theinertial forces dominates.

2. The invention in accordance with claim 1 wherein the Reynolds numberfor flow through the orifice is greater than 100.

3. The invention in accordance with claim 1 wherein means are providedto adapt the timer to a bomb for use as a retardation sensor.

4. The invention in accordance with claim 1 wherein means are providedto adapt the timer for use as a rocket arming delay timer.

5. The invention in accordance with claim 1 wherein the piston issubstantially cylindrical and the piston rate of travel through the gasbeing defined by the following equation:

when the flow in the clearance is neglected, where K is an experimentalconstant, F is the axial force applied to the piston, p is the densityof the gas, A is the area of the orifice, A is the piston cross sectionarea, and dx/dt is the velocity of the piston.

6. The invention in accordance with claim 5 wherein the radius of thepiston is less than about l/2 inch.

inches.

9. The invention in accordance with claim 5 wherein the gas is air.

1. A gas, sharp edge orifice dashpot timer comprising: a cylinder havinga substantially cylindrical interior wall; a piston disposed in saidcylinder and having an outer diameter slightly less that the diameter ofsaid interior wall and containing a sharp edge orifice whereby the majorportion of dashpot gas flow occurs through the sharp edge orifice; a gasin the cylinder through which the piston is adapted to move with therelative flow of the gas being inertial flow through the orifice andflow in lubrication region between the piston and cylinder, whichinvolves Reynolds'' numbers much larger than 1 in the orifice so thatthe viscous effects can be neglected for flow through the orifice andthe inertial forces dominates.
 2. The invention in accordance with claim1 wherein the Reynolds number for flow through the orifice is greaterthan
 100. 3. The invention in accordance with claim 1 wherein means areprovided to adapt the timer to a bomb for use as a retardation sensor.4. The invention in accordance with claim 1 wherein means are providedto adapt the timer for use as a rocket arming delay timer.
 5. Theinvention in accordance with claim 1 wherein the piston is substantiallycylindrical and the piston rate of travel through the gas being definedby the following equation: dx/dt KA/A'' Square Root 2F/ Rho A'' when theflow in the clearance is neglected, where K is an experimental constant,F is the axial force applied to the piston, Rho is the density of thegas, A is the area of the orifice, A'' is the piston cross section area,and dx/dt is the velocity of the piston.
 6. The invention in accordancewith claim 5 wherein the radius of the piston is less than about 1/2inch.
 7. The invention in accordance with claim 5 wherein F/A'' is of avalue of about 0.05 psi to several hundred psi.
 8. The invention inaccordance with claim 5 wherein h, the piston-cylinder clearance, isless than 0.001 inches.
 9. The invention in accordance with claim 5wherein the gas is air.